In recent years, under growing awareness of environmental friendliness, photovoltaic power generation attracts attention. In this situation, various solar cells have been developed, and some of them are practically used. Above all, thin-film silicon solar cells that are small in the amount of silicon used as a raw material and therefore suitable for mass production are expanding in production scale.
However, the thin-film silicon solar cells exposed to light irradiation decline in power generation efficiency with the lapse of time, and this so-called light induced degradation problem is not yet overcome. These solar cells are requested to be further improved to achieve higher energy conversion efficiency.
The amorphous silicon thin-films used in the thin-film silicon solar cells are often formed by a plasma CVD method using silane gas as a raw material. However, during film formation, unwanted materials such as higher silanes and particles secondarily produced in the plasma can be mixed into the film, and those unwanted materials in the film are considered to cause light induced degradation. The unwanted materials mixed in the film increase the Si—H2 bond concentration of the film. There is a correlation between the Si—H2 bond concentration of the film and the light induced degradation degree, and in order to prevent light induced degradation, it is preferred to keep the Si—H2 bond concentration of the film small. Consequently in order to prevent such light induced degradation, any idea is required to be developed for preventing the unwanted materials such as higher silanes and particles from being mixed into the film.
FIG. 13 is a schematic vertical sectional view showing a conventional general plasma CVD apparatus. In a vacuum vessel P1 of a plasma CVD apparatus PA13 shown in FIG. 13, a plasma generating electrode P2 and an earth electrode P23 are disposed to face each other with a gap kept between them. One end of an exhaust pipe P1 is connected with the vacuum vessel P1, and the other end of the exhaust pipe P10 is connected with a gas exhaust device P9. The gas exhaust device P9 is used to keep the pressure in the vacuum vessel P1 low through the exhaust pipe P10. The plasma generating electrode P2 is provided with gas supply ports P16 like a shower head, and a source gas such as silane gas is fed from the gas supply ports 16 into the vacuum vessel P1. The source gas is supplied from a gas introduction pipe P14 into the gas supply ports P16. The earth electrode P23 supporting a substrate P3 is provided with a substrate heating mechanism P11, and the exhaust pipe P10 is provided with a pressure regulating valve P20.
A power supply P6 connected with the plasma generating electrode P2 supplies power to the plasma generating electrode P2, to generate a plasma P7. The generated plasma P7 decomposes the source gas, to generate a film forming species that forms a thin film on the substrate P3 held on the earth electrode P23.
As one method for removing the unwanted materials produced in the plasma processing apparatus, known is a method of sucking and removing the unwanted materials from the gas exhaust holes formed in the plasma generating electrode.
Patent Literature 1 discloses numerous exhaust holes having an inner diameter of not larger than the Debye length, which are distributed to open in the entire surface facing the substrate, of the electrode. Patent Literature 1 does not refer to the removal of unwanted materials by the numerous exhaust holes opening in the surface of the electrode, but the exhaust holes are provided to allow the equal supply of the gas onto the film forming surface and the equal exhaust of the gas from the film forming surface, for realizing uniform plasma processing and uniform film formation.
Patent Literature 2 discloses an electrode having numerous recesses and projections, in which gas supply holes are formed in either the projections or the recesses, while gas sucking holes are formed in the other.
Patent Literature 3 discloses an electrode provided with both gas introduction holes and gas exhaust holes and further having numerous recesses formed on the surface thereof. In the recesses on the surface of the electrode, a highly dense plasma is generated, and the gas introduction holes formed in the recesses are used to highly efficiently decompose the source gas. Further, the gas exhaust holes formed in the recesses allow decomposed species to be removed before the decomposed species produced in the highly dense plasma grow into higher silanes, clusters, etc. owing to reactions.